Elevated CO2 does not affect stem CO2 efflux nor stem respiration in a dry Eucalyptus woodland,but it shifts the vertical gradient in xylem [CO2]

To quantify stem respiration (R_S) under elevated CO₂ (eCO₂), stem CO₂ efflux (E_A) and CO₂ flux through the xylem (F_T) should be accounted for, because part of respired CO₂ is transported upwards with the sap solution. However, previous studies have used E_A as a proxy of R_S, which could lead to equivocal conclusions. Here, to test the effect of eCO₂ on R_S, both E_A and F_T were measured in a free‐air CO₂ enrichment experiment located in a mature Eucalyptus native forest. Drought stress substantially reduced E_A and R_S, which were unaffected by eCO₂, likely as a consequence of its neutral effect on stem growth in this phosphorus‐limited site. However, xylem CO₂ concentration measured near the stem base was higher under eCO₂, and decreased along the stem resulting in a negative contribution of F_T to R_S, whereas the contribution of F_T to R_S under ambient CO₂ was positive. Negative F_T indicates net efflux of CO₂ respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of R_S on eCO₂ and suggest stimulated root respiration under eCO₂ that may shift vertical gradients in xylem [CO₂] confounding the interpretation of E_A measurements.     

Dynamics of soil CO2 efflux under varying atmospheric CO2 concentrations reveal dominance of slow processes

We evaluated the effect on soil CO₂ efflux (F_CO2) of sudden changes in photosynthetic rates by altering CO₂ concentration in plots subjected to +200 ppmv for 15 years. Five‐day intervals of exposure to elevated CO₂ (eCO₂) ranging 1.0–1.8 times ambient did not affect F_CO2. F_CO2 did not decrease until 4 months after termination of the long‐term eCO₂ treatment, longer than the 10 days observed for decrease of F_CO2 after experimental blocking of C flow to belowground, but shorter than the ~13 months it took for increase of F_CO2 following the initiation of eCO₂. The reduction of F_CO2 upon termination of enrichment (~35%) cannot be explained by the reduction of leaf area (~15%) and associated carbohydrate production and allocation, suggesting a disproportionate contraction of the belowground ecosystem components; this was consistent with the reductions in base respiration and F_CO2‐temperature sensitivity. These asymmetric responses pose a tractable challenge to process‐based models attempting to isolate the effect of individual processes on F_CO2.     

Changes in Lipid Metabolism during Adaptation of the Dunaliella salina Photosynthetic Apparatus to High CO2 Concentration

The effects of CO₂ on the content and composition of lipid fatty acids (FA) and on the photosynthetic characteristics of unicellular halophilic green alga Dunaliella salina (known to be susceptible to CO₂ stress) were investigated. It was shown that even one-day-long increase in the CO₂ concentration (from 2 to 10%) provoked an increase in the total amount of FA on the dry weight basis by 30%. After 7-day-long growth at 10% CO₂, this value was 2.7-fold higher than that at 2% CO₂. The difference in the FA content and composition indicated the activation of FA synthesis de novo and inhibition of their elongation and desaturation, as well as the increase in the relative content of saturated FA at 10% CO₂. It was demonstrated that, after one-day-long CO₂ stress, the MGDG/DGDG ratio increased fourfold without change in the sum of their FA, which indicates the increase in the proportion of lipids predisposed to micellar (hexagonal phase) but not lamellar structure formation. Under short-term CO₂ stress, the ratio of 3/6 FA increased and the content of E-16:113 FA in phosphatidylglycerols increased sharply. The drop in protein content especially in the photosystem I (PSI) preparations, as well as diminishing the ratio of F ₇₀₀-to-F ₆₈₆ nm fluorescence (F ₇₀₀/F ₆₈₆) under short-term CO₂ stress argued for the significant damage to PSI. The reversibility of these changes at more prolonged treatment (7 and 10 days) demonstrated that D. salina cells could restore the functional activity of PSI. The lower level of F ₇₀₀/F ₆₈₆, chlorophyll a (Chla)/Chlb, and 3/6 FA ratio in line with the higher level of E-16:113 in the cells growing for a long time at the high CO₂ concentration is characteristic for the new structural and functional state of the photosynthetic apparatus providing for the effective photosynthesis of D. salina under these conditions.     

CO2 enrichment predisposes foliage of a eucalypt to freezing injury and reduces spring growth

Seedlings of Eucalyptus pauciflora, were grown in open-top chambers fumigated with ambient and elevated [CO₂], and were divided into two populations using 10% light transmittance screens. The aim was to separate the effects of timing of light interception, temperature and [CO₂] on plant growth. The orientation of the screens exposed plants to a similar total irradiance, but incident during either cold mornings (east-facing) or warm afternoons (west-facing). Following the first autumn freezing event elevated CO₂-grown plants had 10 times more necrotic leaf area than ambient CO₂ plants. West-facing plants had significantly greater (25% more) leaf damage and lower photochemical efficiency (F_v/F_m) in comparison with east-facing plants. Following a late spring freezing event east-facing elevated CO₂ plants suffered a greater sustained loss in F_v/F_m than west-facing elevated CO₂- and ambient CO₂-grown plants. Stomatal conductance was lower under elevated CO₂ than ambient CO₂ except during late spring, with the highest leaf temperatures occurring in west-facing plants under elevated CO₂. These higher leaf temperatures apparently interfered with cold acclimation thereby enhancing frost damage and reducing the ability to take advantage of optimal growing conditions under elevated CO₂.     

Adaptation to Low CO(2) Level in a Mutant of Anacystis nidulans R(2) which Requires High CO(2) for Growth

The mutant E₁ of Anacystis nidulans R₂ requires high CO₂ concentration for growth but was able to adapt to low CO₂ concentration. This was exhibited by the increased ability to accumulate inorganic carbon within the cells and the large increase in the amount of a 42-kilodalton polypeptide located in the cytoplasmic membrane. The adaptation occurred in E₁ cells at an extracellular CO₂ concentration as high as 0.3%, which was 8 times the concentration for maximal adaptation in R₂ cells. The ability of E₁ cells to exhibit low CO₂ characteristics at a higher CO₂ concentration was attributed to lower intracellular CO₂ concentration.     

Sustained effects of atmospheric [CO2] and nitrogen availability on forest soil CO2 efflux

Soil CO₂ efflux (F_soil) is the largest source of carbon from forests and reflects primary productivity as well as how carbon is allocated within forest ecosystems. Through early stages of stand development, both elevated [CO₂] and availability of soil nitrogen (N; sum of mineralization, deposition, and fixation) have been shown to increase gross primary productivity, but the long‐term effects of these factors on F_soil are less clear. Expanding on previous studies at the Duke Free‐Air CO₂ Enrichment (FACE) site, we quantified the effects of elevated [CO₂] and N fertilization on F_soil using daily measurements from automated chambers over 10 years. Consistent with previous results, compared to ambient unfertilized plots, annual F_soil increased under elevated [CO₂] (ca. 17%) and decreased with N (ca. 21%). N fertilization under elevated [CO₂] reduced F_soil to values similar to untreated plots. Over the study period, base respiration rates increased with leaf productivity, but declined after productivity saturated. Despite treatment‐induced differences in aboveground biomass, soil temperature and water content were similar among treatments. Interannually, low soil water content decreased annual F_soil from potential values – estimated based on temperature alone assuming nonlimiting soil water content – by ca. 0.7% per 1.0% reduction in relative extractable water. This effect was only slightly ameliorated by elevated [CO₂]. Variability in soil N availability among plots accounted for the spatial variability in F_soil, showing a decrease of ca. 114 g C m^?2 yr^?1 per 1 g m^?2 increase in soil N availability, with consistently higher F_soil in elevated [CO₂] plots ca. 127 g C per 100 ppm [CO₂] over the +200 ppm enrichment. Altogether, reflecting increased belowground carbon partitioning in response to greater plant nutritional needs, the effects of elevated [CO₂] and N fertilization on F_soil in this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production.     

Factors affecting exhaled nitric oxide measurements: the effect of sex

Background

Exhaled nitric oxide (F_ENO) measurements are used as a surrogate marker for eosinophilic airway inflammation. However, many constitutional and environmental factors affect F_ENO, making it difficult to devise reference values. Our aim was to evaluate the relative importance of factors affecting F_ENO in a well characterised adult population.

Methods

Data were obtained from 895 members of the Dunedin Multidisciplinary Health and Development Study at age 32. The effects of sex, height, weight, lung function indices, smoking, atopy, asthma and rhinitis on F_ENO were explored by unadjusted and adjusted linear regression analyses.

Results

The effect of sex on F_ENO was both statistically and clinically significant, with F_ENO levels approximately 25% less in females. Overall, current smoking reduced F_ENO up to 50%, but this effect occurred predominantly in those who smoked on the day of the F_ENO measurement. Atopy increased F_ENO by 60%. The sex-related differences in F_ENO remained significant (p < 0.001) after controlling for all other significant factors affecting F_ENO.

Conclusion

Even after adjustment, F_ENO values are significantly different in males and females. The derivation of reference values and the interpretation of F_ENO in the clinical setting should be stratified by sex. Other common factors such as current smoking and atopy also require to be taken into account.     

The effect of acute hypoxia on prostaglandin release in perfused human fetal placenta

The release of prostaglandin E₂ and F_2α, thromboxane B₂ and 6-keto-prostaglandin F_1α was measured in isolated human placental cotyledons perfused under high- and low-oxygen conditions. Also the effect of reoxygenation on prostaglandin production was studied. During the high-oxygen period, prostaglandin E₂ accounted for 44 % and 6-keto-prostaglandin F_1α for 28 % of all prostaglandin release, and the rank order of prostaglandin release was E₂ > 6-keto-prostaglandin F_1α > thromboxane B₂ > prostaglandin F_2α. Hypoxia had no significant effect on quantitative prostaglandin release, but the ration of prostaglandin E₂ to prostaglandin F_2α was significantly increased. After the hypoxic period during reoxygenation the release of 6-keto-prostaglandin F_1α was significantly decreased, as was the ratio of 6-keto-prostaglandin F_1α to thromboxane B₂. Also the ratio of the vasodilating prostaglandins (E₂, 6-keto-prostaglandin F_1α) to the vasocontricting prostaglandins (thromboxane B₂, prostaglandin F_2α) was decreased during reoxygenation period. With the constant flow rate, the perfusion pressure increased during hypoxia in six and was unchanged in three preparation. The results indicate that changes in the tissue oxygenation in the placenta affect prostaglandin release in the fetal placental circulation. This may also have circulatory consequences.     

Uterine vein prostaglandin levels in late pregnant dogs

We studied the uterine venous plasma concentrations of prostaglandins E₂, F_2α, 15 keto 13,14 dihydro E₂ and 15 keto 13,14 dihydro F_2α in late pregnant dogs in order to evaluate the rates of production and metabolism of prostaglandin E₂ and F_2α in pregnancy $\text{in vivo}$. We used a very specific and sensitive gas chromatography-mass spectrometry assay to measure these prostaglandins. The uterine venous concentrations of prostaglandin E₂ and 15 keto 13,14 dihydro E₂ were 1.35±.27 ng/ml and 1.89±.37 ng/ml, respectively; however, we could not find any prostaglandin F_2α and very little of its plasma metabolite in uterine venous plasma. Since uterine microsomes can generate prostaglandin F_2α and E₂ from endoperoxides, prostaglandin F_2α production $\text{in vivo}$ must be regulated through an enzymatic step after endoperoxide formation. Prostaglandin E₂ is produced by pregnant canine uterus in quantities high enough to have a biological effect in late pregnancy; however, prostaglandin F_2α does not appear to play a role at this stage of pregnancy.     

Stomatal responses of Douglas-fir seedlings to elevated carbon dioxide and temperature during the third and fourth years of exposure

Two major components of climate change, increasing atmospheric [CO₂] and increasing temperature, may substantially alter the effects of water availability to plants through effects on the rate of water loss from leaves. We examined the interactive effects of elevated [CO₂] and temperature on seasonal patterns of stomatal conductance (g_s), transpiration (E) and instantaneous transpiration efficiency (ITE) in Douglas‐fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings. Seedlings were grown in sunlit chambers at either ambient CO₂ (AC) or ambient + 180 µmol mol^?1 CO₂ (EC), and at ambient temperature (AT) or ambient + 3·5 °C (ET) in a full‐factorial design. Needle gas exchange at the target growth conditions was measured approximately monthly over 21 months. Across the study period and across temperature treatments, growth in elevated [CO₂] decreased E by an average of 12% and increased ITE by an average of 46%. The absolute reduction of E associated with elevated [CO₂] significantly increased with seasonal increases in the needle‐to‐air vapour pressure deficit (D). Across CO₂ treatments, growth in elevated temperature increased E an average of 37%, and did not affect ITE. Combined, growth in elevated [CO₂] and elevated temperature increased E an average of 19% compared with the ACAT treatment. The CO₂ supply and growth temperature did not significantly affect stomatal sensitivity to D or the relationship between g_s and net photosynthetic rates. This study suggests that elevated [CO₂] may not completely ameliorate the effect of elevated temperature on E, and that climate change may substantially alter needle‐level water loss and water use efficiency of Douglas‐fir seedlings.     

High-light effects on CO2 fixation gradients across leaves

Chlorophyll fluorescence and internal patterns of ¹⁴CO₂ fixation were measured in sun and shade leaves of spinach after treatment with various light intensities. When sun leaves were irradiated with 2000μmol m^?2 s^?1 for 2h, F_V/F_M decreased by about 15%, but ¹⁴CO₂ fixation was unaffected, whereas shade leaves exhibited a 21% decrease in F_v/F_M and a 25% decrease in ¹⁴CO₂ fixation. Irradiation of sun and shade leaves with 4000μmol m^?1 for 4 h decreased F_V/F_M by 30% in sun leaves and 40% in shade leaves, while total ¹⁴CO₂ fixation decreased by 41% in sun leaves and 55% in shade leaves. After light treatment, gradients of CO₂ fixation across leaves were determined by measuring ¹⁴CO₂ fixed in paradermal leaf sections after a 10s pulse of ¹⁴CO₂. Gradients of ¹⁴CO₂ fixation in control sun and shade leaves were identified when expressed on a relative basis and normalized for leaf depth. Treatment of leaves with 2000 μmol PAR m^?2 s^?1 for 2h did not after patterns of carbon fixation across sun leaves, but slightly altered the pattern in shade leaves. In contrast, treatment of sun and shade leaves with 4000μmol m^?2 s^?1 for 4h decreased carbon fixation more in the palisade mesophyll cells than in the spongy mesophyll cells of sun and shade leaves, and fixation in medial tissue of shade leaves was dramatically decreased compared to the adaxial and abaxial tissue. The interaction between leaf anatomy and biochemical parameters involved in tolerance to photoinhibition in spinach is discussed.     

Effects of lifelong [CO2] enrichment on carboxylation and light utilization of Quercus pubescens Willd. examined with gas exchange,biochemistry and optical techniques

Lifelong exposure to elevated concentrations of atmospheric CO₂ may enhance carbon assimilation of trees with unlimited rooting volume and consequently may reduce requirements for photoprotective pigments. In early summer the effects of elevated [CO₂] on carboxylation and light utilization of mature Quercus pubescens trees growing under chronic [CO₂] enrichment at two CO₂ springs and control sites in Italy were examined. Net photosynthesis was enhanced by 36 to 77%. There was no evidence of photosynthetic downregulation early in the growing season when sink demand presumably was greatest. Specifically, maximum assimilation at saturating [CO₂], electron transport capacity, and Rubisco content, activity and carboxylation capacity were not significantly different in trees growing at the CO₂ springs and their respective control sites. Foliar biochemical content, leaf reflectance index of chlorophyll pigments (NDVI), and photochemical efficiency of PSII (ΔF/F_m′) also were not significantly affected by [CO₂] enrichment except that starch content and ΔF/F_m′ tended to be higher at one spring (42 and 15%, respectively). Contrary to expectation, prolonged elevation of [CO₂] did not reduce xanthophyll cycle pigment pools or alter mid‐day values of leaf reflectance index of xanthophyll cycle pigments (PRI), despite the enhancement of carbon assimilation. However, both these pigments and PRI were well correlated with electron transport capacity.     

CO2 uptake and fluorescence responses for a shade-tolerant cactus Hylocereus undatus under current and doubled CO2 concentrations

Hylocereus undatus (Haworth) Britton and Rose growing in controlled environment chambers at 370 and 740 μmol CO₂ mol^?1 air showed a Crassulacean acid metabolism (CAM) pattern of CO₂ uptake, with 34% more total daily CO₂ uptake under the doubled CO₂ concentration and most of the increase occurring in the late afternoon. For both CO₂ concentrations, 90% of the maximal daily CO₂ uptake occurred at a total daily photosynthetic photon flux density (PPFD) of only 10 mol m^?2 day^?1 and the best day/night air temperatures were 25/15°C. Enhancement of the daily net CO₂ uptake by doubling the CO₂ concentration was greater under the highest PPFD (30 mol m^?2 day^?1) and extreme day/night air temperatures (15/5 and 45/35°C). After 24 days of drought, daily CO₂ uptake under 370 μmol CO₂ mol^?1 was 25% of that under 740 μmol CO₂ mol^?1. The ratio of variable to maximal chlorophyll fluorescence (F_y/F_m) decreased as the PPFD was raised above 5 mol m^?2 day^?1, at extreme day/night temperatures and during drought, suggesting that stress occurred under these conditions. F_v/F_m was higher under the doubled CO₂ concentration, indicating that the current CO₂ concentration was apparently limiting for photosynthesis. Thus net CO₂ uptake by the shade-tolerant H. undatus, the photosynthetic efficiency of which was greatest at low PPFDs. showed a positive response to doubling the CO₂ concentration, especially under stressful environmental conditions.     

Energy-dependent quenching of dark-level chlorophyll fluorescence in intact leaves

A new type of modulation fluorometer was used in the study of energy-dependent chlorophyll fluorescence quenching (q_E) in intact leaves. Under conditions of strong energization of the thylakoid membrane (high light intensity, absence of CO₂) not only variable fluorescence, F_V, but also dark-level fluorescence, F_O, was quenched, leading to definition of a quenching coefficient, q_O. Information on q_O was shown to be essential for correct determination of photochemical (q_Q) and energy dependent quenching (q_E) by the saturation pulse method. The relationship between q_E and q_O was analysed over a range of light intensities at steady state conditions. q_E was found to consist of two components, the second of which is linearly correlated with q_O. q_O and the second component of q_E are interpreted to reflect the state 1 — state 2 shift caused by LHC II phosphorylation.     

Basal level of human platelet prostaglandins: PGE1 is more elevated than PGE2

Radioimmunoassays of platelet prostaglandins E₁ and F_1α in platelet rich plasma or platelet suspension, demonstrate that both PGE₁ and PGF_1α are present at higher concentrations than prostaglandins E₂ and F_2α. Gas chromatography — mass spectrometry determinations of prostaglandins E₁ and E₂ in resting washed platelets confirm this difference. Lastly, there is a greater incorporation of [1-¹⁴C] acetate into prostaglandins E₁ and F_1α compared to that into prostaglandins E₂ and F_2α.     

Growth and photosynthesis of two eucalypt species during high temperature stress under ambient and elevated [CO2]

Two species of eucalypt (Eucalyptus macrorhyncha and E. rossii) were grown under conditions of high temperatures (45 °C, maximum) and high light (1500 μmol m^?2 s^?1, maximum) at either ambient (350 μL L^?1) or elevated (700 μL L^?1) CO₂ concentrations for 8 weeks. The growth enhancement, in terms of total dry weight, was 41% and 103% for E. macrorhyncha and E. rossii, respectively, when grown in elevated [CO₂]. A reduction in specific leaf area and increased concentrations of non-structural carbohydrates were observed for leaves grown in elevated [CO₂]. Plants grown in elevated [CO₂] had an overall increase in photosynthetic CO₂ assimilation rate of 27%; however, when measured at the same CO₂ concentration a down-regulation of photosynthesis was evident especially for E. macrorhyncha. During the midday period when temperatures and irradiances were maximal, photosynthetic efficiency as measured by chlorophyll fluorescence (F_v/F_m) was lower in E. macrorhyncha than in E. rossii. Furthermore, F_v/F_m was lower in leaves of E. macrorhyncha grown under elevated than under ambient [CO₂]. These reductions in F_v/F_m were accompanied by increases in both photochemical (qP) and nonphotochemical quenching (qN and NPQ), and by increases in the concentrations of xanthophyll cycle pigments with an increased proportion of the total xanthophyll cycle pool comprising of antheraxanthin and zeaxanthin. Thus, increased atmospheric [CO₂] may enhance photoinhibition when environmental stresses such as high temperatures limit the capacity of a plant to respond with growth to elevated [CO₂].     

Studies of prostaglandins and prostaglandin antagonists on bovine pulmonary vein in vitro

Acetylsalicylic acid (ASA), indomethacin, sodium meclofenamate (FEN), phenylbutazone (PB), phloretin phosphates (PP), SC-19220, and diethylcarbamazine citrate (DECC) were screened against histamine, 5-hydroxytryptamine (5-HT), bradykinin, acetylcholine, and prostaglandins (PG) E₁, E₂, and F_2α to determine their specificity in antagonizing PG's on the bovine pulmonary vein. PG E₂ relaxed the smooth muscle preparation at low concentrations and induced contraction at higher concentrations. PG E₁ consistently evoked dose-related relaxations, whereas PG F_2α contracted the bovine pulmonary vein. Studies with inhibitors suggest that the different actions of prostaglandins could be mediated through different receptors. Sodium meclofenamate and PP dimer blocked PG E₂-induced contractions, whereas relaxations were not blocked. DECC inhibited the relaxant effect of PG E₂. DECC also antagonized histamine, 5-HT, and PG F_2α, suggesting the drug is rather non-specific. Phenylbutazone antagonized the actions of both PG E₂ and PG F_2α on the bovine pulmonary vein. By classifying receptors by antagonism the bovine pulmonary vein appears to contain PG E₂ (PP-type), PG E₂ (FEN-type), PG E₂ (PB-type), and PG F_2α (PB-type) receptors. An absence of SC-type PG-receptors is noted.     

Woody tissue photosynthesis reduces stem CO2 efflux by half and remains unaffected by drought stress in young Populus tremula trees

A substantial portion of locally respired CO₂ in stems can be assimilated by chloroplast-containing tissues. Woody tissue photosynthesis (P_wt) therefore plays a major role in the stem carbon balance. To study the impact of P_wt on stem carbon cycling along a gradient of water availability, stem CO₂ efflux (E_A), xylem CO₂ concentration ([CO₂]), and xylem water potential (Ψ_xylem) were measured in 4-year-old Populus tremula L. trees exposed to drought stress and different regimes of light exclusion of woody tissues. Under well-watered conditions, local P_wt decreased E_A up to 30%. Axial CO₂ diffusion (D_ax) induced by distant P_wt caused an additional decrease in E_A of up to 25% and limited xylem [CO₂] build-up. Under drought stress, absolute decreases in E_A driven by P_wt remained stable, denoting that P_wt was not affected by drought. At the end of the dry period, when transpiration was low, local P_wt and D_ax offset 20% and 10% of stem respiration on a daily basis, respectively. These results highlight (a) the importance of P_wt for an adequate interpretation of E_A measurements and (b) homeostatic P_wt along a drought stress gradient, which might play a crucial role to fuel stem metabolism when leaf carbon uptake and phloem transport are limited.     

Controls of Evapotranspiration and CO2 Fluxes from Scots Pine by Surface Conductance and Abiotic Factors

Evapotranspiration (E) and CO₂ flux (F_c) in the growing season of an unusual dry year were measured continuously over a Scots pine forest in eastern Finland, by eddy covariance techniques. The aims were to gain an understanding of their biological and environmental control processes. As a result, there were obvious diurnal and seasonal changes in E, F_c, surface conductance (g_c), and decoupling coefficient (Ω), showing similar trends to those in radiation (PAR) and vapour pressure deficit (δ). The maximum mean daily values (24-h average) for E, F_c, g_c, and Ω were 1.78 mmol m⁻² s⁻¹, −11.18 µmol m⁻² s⁻¹, 6.27 mm s⁻¹, and 0.31, respectively, with seasonal averages of 0.71 mmol m⁻² s⁻¹, −4.61 µmol m⁻² s⁻¹, 3.3 mm s⁻¹, and 0.16. E and F_c were controlled by combined biological and environmental variables. There was curvilinear dependence of E on g_c and F_c on g_c. Among the environmental variables, PAR was the most important factor having a positive linear relationship to E and curvilinear relationship to F_c, while vapour pressure deficit was the most important environmental factor affecting g_c. Water use efficiency was slightly higher in the dry season, with mean monthly values ranging from 6.67 to 7.48 μmol CO₂ (mmol H₂O)⁻¹ and a seasonal average of 7.06 μmol CO₂ (μmol H₂O)⁻¹. Low Ω and its close positive relationship with g_c indicate that evapotranspiration was sensitive to surface conductance. Mid summer drought reduced surface conductance and decoupling coefficient, suggesting a more biotic control of evapotranspiration and a physiological acclimation to dry air. Surface conductance remained low and constant under dry condition, supporting that a constant value of surface constant can be used for modelling transpiration under drought condition.